What is parallel query execution?

Parallelism allows SQL Server to divide parts of a query across multiple worker threads so work can be performed concurrently on multiple CPU schedulers.

Parallel plans can improve performance for expensive analytical queries, but parallelism also has coordination overhead. Operators such as Distribute Streams, Repartition Streams, and Gather Streams appear in parallel plans.

Whether a query goes parallel depends on estimated cost, server configuration, query hints, available resources, and optimizer decisions.